Ecophysiological studies on four selected succulent Karoo species

[摘要] ENGLISH ABSTRACT: A study was undertaken in three greenhouses of differing temperature regimes (low,intermediate and high), to determine the ecophysiological plasticity of four selectedSucculent Karoo shrub species. The temperature acclimation potential was determinedfor species grown under different ambient temperature treatments and seasonaltemperature regimes. The four study species were Zygophyllum prismatocarpum andStoeberia utilis (from the Namaqualand-Namib Domain) as well as Tripteris sinuataand Ruschia spinosa (from the Little Karoo ). All the study species are leaf succulentshrubs, except for Tripteris sinuata, which is a drought-deciduous shrub. Theacclimation potential of the species was investigated with regard to the following:photosynthetic and transpiration rates, stomatal conductance, photosynthetictemperature optima, photosynthetic mode (C3 or CAM), instantaneous water-useefficiency, vegetative growth and flowering phenology.Gas exchange measurements performed on the evergreen species, Zygophyllumprismatocarpum, and the drought-deciduous species, Tripteris sinuata, indicated thatboth species are capable of photosynthetic acclimation when grown under differentgrowth and seasonal temperature regimes. Z. prismatocarpum generally obtained thehighest photosynthetic rates in the high temperature treatment and in September 1997(spring). T. sinuata obtained highest photosynthetic rates in the intermediate and hightemperature treatments and in June 1997 (winter). T. sinuata had much higherphotosynthetic rates compared to Z. prismatocarpum. Photosynthetic temperatureoptima of Z. prismatocarpum and T. sinuata plants increased with an increase ingrowth and seasonal temperature regimes and differed between species. Temperatureoptima of Z. prismatocarpum plants varied from 21-24°C in spring and from 28-32°Cin summer. T. sinuata plants had temperature optima ranging from 24-28°C in springand from 28-33°C in summer.Leaf stable carbon isotope composition (813C) varied between temperature treatments,seasons and species. All four species studied had 813C values indicating the C3photosynthetic mode in both winter and summer, ranging from -24 to -31 %0. Lessnegative 813C values (higher water-use efficiency) were obtained for plants grown inthe high temperature treatment, regardless of season, and during summer, regardlessof greenhouse temperature regime. S. utilis and R. spinosa had less negative 813Cvalues compared to Z prismatocarpum and T sinuata plants. Vegetative growthmeasurements (leaf area and plant volume) indicated continuous growth for Zprismatocarpum and R. spinosa plants, regardless of treatment temperature. Bycontrast, vegetative growth was greatest for T sinuata and S. utilis plants in autumnand winter, and in spring and summer respectively. The study species showed greatplasticity in their flowering phenology, especially for T sinuata plants. T sinuataplants showed mass flowering in the high temperature treatment, especially duringsummer. S. utilis plants grown in the low temperature treatment flowered in autumnand early winter and plants in the high temperature treatment flowered later in winter.R. spinosa plants only flowered in autumn in the low temperature treatment and inspring in the intermediate temperature treatment. Z prismatocarpum plants did notflower at all.Plants of Z prismatocarpum and S. utilis, grown under the same temperature regime,were used in a water stress experiment. The plants were grown under three watertreatments: short-term water stress, long-term water stress and non-stressed (control).Predawn acidity measurements of S. utilis leaves grown under different water stresstreatments indicated increasing levels of titratable acidity in long-term stressedcompared to non-stressed plants. This indicates the possibility that S. utilis plantsfrom the long-term water stress treatment, are facultative CAM/C3.Z prismatocarpum and S. utilis plants from the short-term water stress treatmentresponded very rapidly to re-watering.The predicted increase in global mean temperature by approximately 2-4°C during thenext century requires an understanding of plant responses to temperatures abovecurrent ambient temperatures. As an important ancillary objective, this studyattempted to improve our understanding of the responsiveness of succulent Karooshrubs to changing temperatures, and to provide a framework to facilitate theprediction of the responses of Karoo plants to future climatic scenarios. The studyindicated that Z. prismatocarpum and S. utilis plants are capable of photosyntheticacclimation when grown under different temperature regimes. It is thus suggestedthat these plants are capable of photosynthetic acclimation to a predicted future globaltemperature increase. Water-use efficiency studies of the four study species alsosuggest that, these species as well as possible other succulent Karoo species, willincrease their water-use efficiency in response to a predicted climate change. Withhigher global temperatures, the study also suggests that succulent plants would relymore on CAM as a means of conserving water to tolerate drought stress.

[发布机构] Stellenbosch University